1. Field of the Invention
The present invention relates to creating images by techniques including a thermal imaging technique, and, more particularly, relates to a system, apparatus, computer program product and/or method for enhancing color uniformity of images produced by way of a printer having multiple thermal print heads.
2. Description of Prior Art
Many people enjoy reading the “Parade” section of their local Sunday newspaper and are usually enticed by the multicolored pictures on its first cover page. If one were to apply a magnifying glass to those pictures, the underlying dot patterns from which those pictures are composed would be readily discernable. This process of composing pictures by dot patterns is well established. Such patterns can be rendered by various methods including traditional offset printing, and digital imaging techniques such as electrophotographic, ink-jet and thermal imaging processes. In digital photography, similar dot patterns are used to create images.
Technology underlying the rendition of digitally-photographed images, i.e., the permanent recording of images on paper or on similar substrate material, is continually evolving. As one example of such current technology, reference may be made to U.S. patent application Ser. No. 09/872,424, filed May 30, 2001, assigned to the assignee of the instant application, entitled: “A High Speed Photo-Finishing Apparatus”, having co-inventors S. J. Telfer, M. L. Reisch, A. Bouchard, S. B. Lawrence, B. D. Busch, and M. S. Viola, (now abandoned and replaced by U.S. patent application Ser. No. 10/080,883, filed Feb. 22, 2002 and issued as U.S. Pat. No. 6,842,186 ) which, along with all of its incorporated-by-reference patents [5,694,484; 6,069,982; 6,128,415; 5,809,164; 4,385,302; 4,447,818; 4,540,992; 5,285,220; 5,711,620; 5,569,347; 5,521,626; 5,897,254; 4,686,549; and 5,244,861] and patent applications, is hereby incorporated by reference herein in its entirety.
One technique used in producing “pictures” from digital photography is thermal imaging. In one process for thermal imaging, a thermal print head containing a single column of a number of linearly-disposed thermal print head heating elements can be used. The elements are pressed against the back side of an ink donor ribbon or tape which, in turn, has its ink side pressed against an ink-receptor substrate, which may be paper (or a material having similar reflective properties to paper) bearing a suitable coating for receiving the ink or dye. The two substrates are moved in a direction perpendicular to the column of elements, which are heated by electrical pulses and which cause the ink to liquefy at various points of contact between each element and the donor ribbon corresponding to the occurrence of the pulses. (Hereinafter vertical formations shall be termed “columns” which are defined perpendicular to direction of substrate motion, and horizontal formations shall be termed “rows” which are defined parallel to direction of substrate motion.) The liquefied ink from the donor ribbon is then registered as dots onto the receiver substrate against which the donor ribbon is being pressed. The image is formed as an array of dots (pixels) in the color of the donor ribbon's ink color. Variation in level of color in the image may be achieved by means of two possible methods. In the first method, the area coverage of dye is approximately constant over the whole area of the pixel, and the amount of dye (the dye “density”) of approximately constant coverage varies according to the amount of energy supplied by the print head to that particular pixel. This method is hereinafter referred to as “variable density” printing, and is commonly practiced in the thermal transfer imaging technique known as “dye diffusion thermal transfer”, or D2T2. In the second method, the size of dots within the area of one pixel varies according to energy supplied by the print head, these dots containing only a single density of dye (de facto, its maximum density). The dots are so small that they cannot be individually distinguished by the naked eye, and so the overall level of color is perceived as an average of the almost total absorption of light in the proportion of the viewed area occupied by dots, and the almost complete (diffuse) reflection of light in unprinted areas. This technique of thermal transfer printing is known hereinafter as “variable dot” printing. A particularly preferred method for variable dot imaging is disclosed in U. S. patent application Ser. No. 09/745,700, filed Dec. 21, 2000, entitled: “Thermal Transfer Recording System”, having co-inventors Michael J. Arnost, Alain Bouchard, Yongqi Deng, Edward J. Dombrowski, Russell A. Gaudiana, Fariza B. Hasan, Serajul Hague, John L. Marshall, Stephen J. Telfer, William T. Vetterling and Michael S. Viola, now U.S. Pat. No. 6,537,410 and in U. S. Provisional patent application Ser. No. 60/294,528, filed May 30, 2001, entitled: “Thermal Mass Transfer Imaging System”, having co-inventors Edward P. Lindholm, Stephen J. Telfer and Michael S. Viola, (the benefit of which was claimed in U.S. patent application Ser. No. 10/159,871, filed May 30, 2003, now U.S. Pat. No. 6,761,788) both of which are assigned to the assignee of the instant application, and both of which, along with all of their incorporated-by-reference patents and patent applications, are hereby incorporated by reference herein in their entireties.
In order to create a “color picture”, multiple colors are needed (typically, the three subtractive primary colors—cyan, magenta, and yellow, although other colors, e.g., black, may be added). In variable dot printing, registration of dots of the three different colors with respect to one another can influence the visual appearance of the picture. In one common practice, known hereinafter as “dot-on-dot” printing, wherein the printing system uses only one thermal print head, a single donor ribbon containing three separate donor colors in repetitive sequence is used in a predetermined order: e.g., cyan, magenta, yellow (followed by a protective clear overcoat). In this process, a repetitive reciprocation of the media must be used to first apply dots of cyan color, then dots of magenta color superimposed exactly on the first dots, and then dots of yellow color superimposed exactly on the dots of the first two colors. As described above, by varying size of each deposition of each color on each dot, one can create a visual image to the naked eye in which dots are invisible and the resulting effect is a multicolored picture when viewing all dots at once. Dot patterns can have densities of approximately 300 by 300 dots per inch (dpi), with dot size ranging in size from approximately 10 to approximately 100 microns. Shape of the dots may vary, but they are commonly substantially circular. A disadvantage of using only one print head is the requirement for multiple passes under the same head (reciprocation) of the receiver substrate containing the dot patterns, which is cumbersome and time consuming. This disadvantage is somewhat offset by the advantage that registration of the dots is relatively straightforward, since the same mechanical system is used to print all three colors. Therefore, a single print head machine can be advantageously used for small, portable tasks, providing good dot-on-dot registration. But, to take advantage of the speed afforded by the preferred method for variable dot printing, one would need to utilize a multiple-head machine.
In a multiple thermal print head machine (tandem printer) each print head is fed its own single-colored donor (i.e., the machine is equipped with a cyan-printing head, a magenta-printing head, and a yellow-printing head). This allows a single pass in one direction of the receptor substrate, greatly speeding up the printing process. However, in this case the three colors are not printed by the same mechanical system, and as a result errors in registration between the colors may occur. Sources of error include but are not limited to: (1) the misalignment of the print heads relative to one another in directions parallel and perpendicular to the motion of the receiver substrate; (2) a skew (angular) misalignment between the printheads; (3) a direction of tracking of the receiver substrate that is not exactly perpendicular to the printheads (or any mistracking or wandering of the receiver substrate); (4) a stretching of the receiver between print heads, which will be influenced by the tension in the receiver between print heads, and which may differ between one pair of print heads and another; (5) a “bag” or “sag” in the receiver between print heads, which will also change the distance between print heads, and may particularly occur in a curved or arcuate receiver path; (6) any roller eccentricities in the receiver path, which will change the effective distance between print heads; and, (7) other mechanical problems, such as slippage of the receiver substrate in the drive mechanism and motor irregularities. Moreover, changes in registration may occur as a result of changes in the above variables with environmental changes, particularly ambient temperature. Gross alignment (i.e., to within about one pixel spacing) of the colored dots in the direction of motion of the substrate is usually not a serious problem because misalignments in that direction can be compensated by varying the time interval, for example, between excitations of the first head's heating elements and excitations of the second and subsequent heads' heating elements. With the substrate moving at constant velocity, time interval variation can accommodate and compensate for distance variation. Likewise, correction of certain of the above-mentioned other problems at the level of the pixel spacing may be achievable with good mechanical design. Unfortunately, however, the level of dot registration required for good image quality in variable dot imaging, using the “dot-on-dot” technique, is far more stringent than one pixel spacing. In fact, as discussed below, registration on the order of a few micrometers is necessary for this technique.
When operating under constraints of a “dot-on-dot” technique, even slight misregistration of superimposed dots can be visually noticeable at least as a color shift or a lack of uniformity of color. This color nonuniformity can be noticed as an erroneous variation in color across a single image (e.g.: an object of uniform color that appears gray-brown on the left-hand side of an image may appear to be blue-brown at the right-hand side of the image). Alternatively, or additionally, this color nonuniformity can be noticed as an erroneous variation in overall color tone from a first print of an image to succeeding prints of the same image.
The reason for this problem in color uniformity can be explained from the physics underlying absorption and reflectivity of different wavelengths of light. Visible light is electromagnetic radiation having wavelengths in the range of approximately 400-700 nanometers. The three so-called primary colors are red, green and blue. Light having wavelengths of approximately 400-500 nanometers appears blue, light of wavelengths of approximately 500-600 nanometers appears green, and light of wavelengths of approximately 600-700 nanometers appears red. Dyes and pigments are materials which selectively absorb certain wavelengths of visible light, and transmit the rest. Yellow dyes absorb blue light, magenta dyes absorb green light, and cyan dyes absorb red light. Black dyes absorb across the whole visible spectrum. When a paper printed with dyes is illuminated, the light which is not absorbed by the printed dyes is diffusely reflected back towards the viewer. The appearance of different colors arises from the subtraction of differing proportions of light at different visible wavelengths.
The color shifts that stem from dot misregistration are a result of overlap in the absorption spectra of the dyes. To appreciate this, consider a surface area “A” half of whose area is completely covered by two ink dye colors with a first dye covering half of area A placed congruently upon a second dye also covering the same half of area A. The first ink dye almost completely absorbs light of red wavelengths but also absorbs a small proportion (for example, 10%) of light of green wavelengths. The second ink dye completely absorbs light of green wavelengths but also absorbs a small proportion (for example, 20%) of light of red wavelengths. Under this condition, which is an example of a “dot-on dot” printing using dyes whose absorption spectra overlap, half of the red and green light impinging on area A is absorbed, and half is reflected. This is because all red and green light impinging on the half of area A bearing the dyes is absorbed, regardless of the overlap of the dye spectra (i.e. no more than all of the red light can be absorbed and no more than all of the green light can be absorbed), whereas none of the red or green light impinging upon the half of area A which bears no dye is absorbed. But, what happens if superimposed dots meander from their “bulls-eye” position so that there is only partial overlap, or even no overlap at all with just side-by-side positioning?
To answer that question, consider the opposite extreme, the side by side case, where both dots now comprise area A, each dot covering 50% of area A. The total amount of green light reflected from Area A is in this case 45% and the total amount of red light reflected from Area A is 40%. [These reflections are based on reducing the otherwise 50% reflection amounts by amounts equal to 10% of 50% A and 20% of 50% A respectively.] The result is twofold: First, area A looks darker than in the “dot-on-dot” case, because the total reflected light is reduced in intensity from 50% reflected red and green light to 40% reflected red and 45% reflected green light. Second, the combined effect of the now-unequal absorbed-red light and absorbed-green light contributions produce a different color from that perceived when the proportions of absorbed red and green light were equal. An exact “dot-off-dot” pattern, or an exact “dot-on-dot” pattern, would each produce a uniform perceived color, although the two colors would be different. But misalignment of a dot-on-dot pattern, or a dot-off-dot pattern, such that in some areas of the image the dots were registered perfectly, and in other areas of the image the dots were entirely non-overlapping, would be manifest as a color-variation in the image.
This problem is further described in, for example, “An Investigation of Color Variation as a Function of Register in Dot-On-Dot Multicolor Halftone Printing”, Jang-fun Chen, M. S. Thesis, Rochester Institute of Technology, 1983, in which it is stated: “It has been observed that color reproduced with dot-on-dot method is extremely sensitive to minute variation in register”. Therefore, the generally unavoidable spectral overlap of various dyes used in dot patterns (regardless of how the patterns were created—by thermal imaging, by ink-jet, by printing press, etc.) can cause an imaging problem in the case of imperfect registration of identical dot patterns of different colors. Superimposed dot patterns that misregister, or meander from “dot-coincidence” to “dot-miss” in an unguided or unsupervised manner because of the above-noted mechanical misalignment with regard to tandem thermal print head machines, or because of other factors with regard to other printing schemes, create color and intensity variations in an image which are perceptible to the naked human eye. The printing press arts attempt to deal with this problem by rotating orthogonal screens, screens having equal resolution in both orthogonal directions, rotating one screen color relative to the other to shift the interference pattern to a high spatial frequency not visible to the naked eye. In the particular case of thermal printing using more than one linear resistor array, wherein each such array has the same fixed dot spacing, misalignment of dots in a direction perpendicular to the direction of motion of the receiver substrate is almost inevitable and can currently be corrected only by measurement of the misalignment and mechanically repositioning one or more of the print heads. Obviously, this approach is cumbersome.
Since the photographic industry is moving rapidly to digital camera photography, since printing speed is a very important factor in producing photographs at retail locations, and since the preferred variable dot thermal transfer printing technology referred to above will allow a fast printing speed of digital images, there is strong motivation to overcome any outstanding difficulties with the technique. Accordingly, there is a need to find a solution to the serious mechanical misalignment problem that arises when using a tandem printer with multiple print heads. Embodiments of the present invention present welcome solutions to these problems of the prior art.